It is well known that there is a steadily increasing demand for higher performance materials in optical applications. In many cases, these materials must be high quality single crystals of a size sufficiently large so that they are capable of being cut, shaped and polished into pieces several millimeters on a side. This is particularly true for solid state optical devices such as all solid state lasers and optical switching devices. For example, there has recently been a rapidly expanding application of new crystals finding use in diode pumped sold state lasers.
These all-solid state lasers typically use a diode laser to pump a solid crystal containing an activator ion to achieve population inversion. This excited ion emits an appropriate coherent wavelength or laser, creating a diode pumped solid state laser (DPSSL). Fully solid state lasers are desirable because they generally are compact, reliable, rugged, and have low power and cooling demands.
Most commonly, a solid state laser crystal will consist of a typical oxide host such as yttrium aluminum garnet (YAG) containing an activator ion such as Nd3+ included in relatively low quantities (≦1%) within the crystal lattice. In traditional Nd: based lasers such Nd:YAG lasers, the Nd3+ ion is pumped by a flash lamp into an absorption band near 808 nm. This populated state relaxes through a non-radiative pathway to a lower energy level that subsequently emits coherent radiation (lasing). In the specific case of Nd3+ activated material, it generally emits a laser wavelength around 1064 nm. Flash lamps have inherent limitations because they have lifetimes of only several hundred hours, require large amounts of energy, emit enormous amounts of waste heat and require large amounts of cooling water. Thus many newer laser systems replace the flash lamp with a diode laser as the pumping source. A diode pumped source is desirable because it creates a much simpler, smaller and more reliable laser platform. DPSSLs require much less energy and produce much less waste heat than the traditional flash lamp pumped lasers. In addition, diode pumps provide wall plug sources over many thousand hours and require only air-cooling.
The most common source of gain medium of DPSSLs is Nd doped YAG produced by the Czochralski pulling technique. There are several problems with the traditional Nd:YAG as gain medium in DPSSLs. The primary limitation for Nd:YAG host is that the ligand field environment in YAG is such that the absorption band for the 808 pumping band is relatively narrow. Unfortunately, the emission wavelength for diode lasers tends to shift with changes in time and temperature. With a narrow absorption manifold around 808 nm, the excitation wavelength of the pumping diode can gradually shift away from the ideal pumping frequency of 808 μm. As this frequency moves away from the narrow absorption band, less energy from the pumping diode is absorbed by the Nd3+ ion. This can lead to a loss in pumping efficiency over time that can severely reduce the performance of the laser. This problem has limited the full-scale implementation of DPSSLs.
To address the above shortcomings other single crystal hosts have been developed for DPSSLs. These include other gain media, especially Nd:YVO4. The most promising host that has emerged for DPSSLs is yttrium orthovanadate YVO4 (commonly called simply “vanadate”). This material forms in the tetrahedral space group I41/amd, and has a crystal structure that is completely different from the garnet structure of YAG. This alternative coordination environment for the Nd3+ activator ion creates a much broader absorption manifold around the 808 nm pumping region. Thus, any gradual change in emission wavelength of the diode pump source does not result in any significantly decreased absorption or decreased pumping efficiency. Therefore, the Nd:YVO4 crystal type has the highest gain coefficient and lowest threshold of the common DPSSL laser crystals. It has a three times larger cross section, shorter lifetime and a larger absorption coefficient than Nd:YAG, making Nd:YVO4 the laser gain medium crystals of choice for DPSSL devices. Nd:YVO4 based DPSSL's have demonstrated high power performance with repetitions of nearly 160 GHz.
Recently other vanadates in single crystal form have emerged as useful DPSSL materials. In particular, Nd:GdVO4 crystals and Nd:GdxY(1-x)VO4 can be grown in the same structure as the corresponding Nd:YVO4 crystals. They show several characteristics that are superior to Nd:YVO4 in crystal DPSSL lasers. In particular, the gadolinium containing crystals with formulas like Nd:GdVO4 have significantly higher thermal conductivity than Nd:YVO4 crystals. Thus, any waste heat can be more easily removed by an appropriate heat sink, reducing any deformation or distortion due to excess heat buildup such that crystals with these formulations can be used for lasers with higher power outputs. Nd:GdVO4 crystals have displayed conversion efficiencies as high as 55% with 14 W output and 62% slope efficiencies. These are considerably higher than any corresponding Nd:YVO4 crystals.
Pure undoped YVO4 has several other attractive characteristics as well. In particular, it is a highly birefringent material with a value of Δn=0.204. Thus, it can be used as an alternative to calcite in polarizers and related applications.
Unfortunately, the crystal growth of either doped or undoped YVO4 crystal is problematic. The material melts incongruently at 1860° C. Thus, it decomposes before it melts, so single crystals of the pure material cannot be grown by traditional Czochralski techniques like Nd:YAG. In addition, the material suffers several other inherent limitations at high temperature. Above about 1000° C. the lattice host material YVO4 begins to extrude vanadium oxide (V2O5). This leads to lattice defects and chemical non-stoichiometry, both of which severely reduce the performance of the laser crystal. Most importantly, at high temperatures, the pentavalent vanadium (V5+) of the host material becomes reduced to V4+ or V3+. These reduced metal sites absorb light strongly and severely degrade the performance of the laser crystal. It should be noted that all of these harmful effects are inherent in the high temperature crystal growth process. The only way to completely eliminate these deficiencies is to lower the temperature at which the crystals are grown.
There have been several attempts to grow these vanadates by other methods, such as, primarily, flux growth, floating zone and top seeded solution growth. These techniques lead to single crystals of YVO4 and Nd:YVO4. However, in all these cases the growth temperature still exceeds 1100° C. and numerous defects due to non-stoichiometry and reduced vanadium ions are still present. Specifically, all heretofore available synthetic methodologies have had considerable shortcomings, leading to defects, inhomogeneities, and “c-axis wander” all of which lead to decreased performance.
Hydrothermal techniques are an excellent route to high quality single crystals for electro-optic applications. For example, all electronic grade quartz is grown commercially by the hydrothermal method. Further, KTP is grown by both flux and hydrothermal methods, and it is widely acknowledged by those skilled in the art that the hydrothermally grown product is of generally superior quality. The hydrothermal method involves the use of superheated water (liquid water heated above its boiling point) under pressure to cause transport of soluble species from a nutrient rich zone to a supersaturated growth zone. Generally a seed crystal is placed in the growth zone. The growth and supersaturation control is achieved by the use of differential temperature gradients. The superheated fluid is generally contained under pressure, typically 5–30 kpsi, in a metal autoclave. Depending on the chemical demands of the system the autoclave can be lined with a nobel metal using a either fixed or floating liner. These general techniques are well known in the art and have been used for the growth of a variety of other electro-optic crystals.
There have been several earlier publications that describe one method of hydrothermal crystal growth of Ln:MVO4, (Ln=Nd, Eu, M=Y, Gd). However, all of these earlier procedures describe hydrothermal growth in strongly acidic solution with pH values below pH=0.15. These earlier reports clearly state that solutions with high pH or alkaline solutions are not usable as they lead to formation of Y(OH)3 instead of the desired material. In addition, the previous reports involve temperatures below 300° C. These reports state that solubility in these strongly acidic solutions decreases steadily from 180° C. to 300° C. (negative solubility) and state explicitly “. . . that above 300° C. the solubility does not change much, which suggests that crystal growth beyond this temperature is not practical”. Most importantly, these previous reports specifically describe only the spontaneous nucleation of very tiny crystals completely unusable for any current device application as described above. There is no mention of transport growth to seeds for the production of large crystals that are useful for device applications. (See, K. Byrappa, T. Ohachi (Eds.) Crystal Growth Technology, William Andrew Pubs, 2002 Chapter 10 pp 335–363; K. Byrappa, B. Nirmala, Ind. J. Phys 73A(5) (1999) pp 621–632)